The present invention relates to elastic surface wave transducers.
The elastic surface wave devices generally have two sets of electrodes in the form of interdigitated transducers deposited on the surface of a piezoelectric substrate. An electrical voltage which is to be processed (for example filter or delay) is applied to one of the sets, which is called the transmission transducer in order to bring about the transmission of elastic surface waves in the direction of the other set, which is called the reception transducer and which supplies a voltage, whose characteristics are dependent on the vibratory mode transmitted and the configuration of the transducer elements.
One of the important characteristics of an elastic surface wave device is its frequency response, which is the product of the responses of the two transducers. It is known that the envelope of the pulse response of a transducer is reproduced in the design formed by the ends of the fingers of at least one of the electrodes of the transducer. Thus, for example, a band pass filter is obtained when one of the two transducers has fingers of unequal length describing a curve representing the sin X/X function and at least its central lobe and its minor main lobes. The central frequency is defined by the spacing between two successive fingers of the same transducer and the band width is linked with the transducer length. The transfer function of the filter is however linked with other parameters which introduce parasitic effects with respect to which it is difficult to take action. One of these effects is constituted by the reflections of surface waves on successive fingers. These reflections introduce transmission losses, which can vary as a function of the frequency leading to deformations in the pass band. It is known to neutralise the waves retransmitted at each finger in this way by using split-finger transducers in such a way that the successive reflected waves are in phase opposition and therefore cancel one another out, instead of being added to one another as is the case with solid fingers. However, this neutralisation does not prevent transmission losses if the reflection coefficient is high. It has been experimentally found that the reflection coefficient depends to a significant extent on the load impedance of the two transducers. For example, in the case of a transmission transducer, this impedance is constituted by the internal impedance of the circuit to which the particular surface wave device is connected. According to experimental results, when this load impedance is low, the reflection coefficient is low for split-finger transducers, but high for solid finger transducers. However, when the load impedance is high the reflection coefficient is high with split finger transducers.